Dendritic cells (DCs) help to coordinate and bridge innate and adaptive immune responses. DCs for the most part are short lived;how long they live and where may influence whether they induce immunity, tolerance or autoimmune disease. DC fate and cytokine production are regulated in part by RANK (receptor activator ofNF-IcB ligand), receptors for TRAIL (tumor necrosis factor-alpha-related papooses-inducing ligand) and the soluble 'decoy'receptor osteoprotegerin (OPG), which binds RANK ligand (RANKL) and TRAIL. Our data suggest that DC cytokine production and survival are dysregulated in the absence of OPG. We plan to define the role the RANKL/RANK/OPG system plays in regulating DCs. We wiU test the hypothesis that OPG regulates dendritic cell survival by modulating the RANKL/RANK and TRAIL/TRAIL receptor pathways. We will test whether OPG -/- or RANK -/- DCs differ from wildtype DCs in their survival in vitro or in vivo. The effect of overexpressing the survival protein Bcl-2 on the DC- restricted CD1 lc promoter will be evaluated in OPG -/- and RANK -/- mice. We predict that by altering the lifespan of DCs in vivo, that underlying defects in disease, bone homeostasis and peripheral lymphoid development will be altered. We will also test the hypothesis that human immature DCs (iDCs) and mature DCs (mDCs) differ in how they are regulated by the RANK/OPG/TRAIL receptor pathways and by caspases. Next we will test the hypothesis that OPG regulates the profile and levels of cytoldnes produced by DC subsets. We will define how the absence of OPG or RANK affects cytokine production in vitro and test whether the cytokine dysregulation in OPG -/- and RANK -/- mice influences in vivo responses to lipopolysaccharide (LPS) and peptide-induced experimental autoimmune encephalomyelitis (EAE). The expression of OPG, RANKL and TRAIL receptors is regulated by 17-13-estradiol (E2). Therefore, we will test the hypotheses that E2 modulates peptide-induced EAE, DC survival and cytokine production via an OPG-<lependent pathway. If OPG is required for E2 to downregulate EAE, we will explore if this is due in part to direct effects of E2 on DCs. Further understanding of how DC longevity and cytokine secretion are regulated may lead to new insights into the causes of and treatments for chronic immunologic diseases like rheumatoid arthritis and multiple sclerosis. The proposed studies may also contribute to understanding of how E2 and OPG contribute to the development of inflammatory immune responses.